Commonmetal pipe fittings such as an elbow, a tee, a cross, etc., are generally provided with female threads for screwing with male threads at the ends of metal pipes. This kind of structure for uniting metal pipes is quite inconvenient as pipes often have to be cut to a proper length and male threads have to be newly made. This takes time and work, especially if the pipes should be brought back to a work place from the site for thread cutting. Besides, the thickness of the pipe should become thinner and the surface may be broken after the male thread is cut on the pipe, so the threaded part may easily be corroded and brlken after a period of use. In addition, a metal pipe used for high heated liquid can expand, so an aperture can be formed at a pipe fitting letting the liquid in the pipe leak out. Above all, a stainless steel pipe is widely used for transporting hot water, but is has a rather large rate of expansion, so its fittings are liable to break and cause leakage after a long period of use.